- Yang, Yi;
- Hoeflich, Peter;
- Baade, Dietrich;
- Maund, Justyn R;
- Wang, Lifan;
- Brown, Peter J;
- Stevance, Heloise F;
- Arcavi, Iair;
- Burke, Jamison;
- Cikota, Aleksandar;
- Clocchiatti, Alejandro;
- Gal-Yam, Avishay;
- Graham, Melissa L;
- Hiramatsu, Daichi;
- Hosseinzadeh, Griffin;
- Howell, D Andrew;
- Jha, Saurabh W;
- McCully, Curtis;
- Patat, Ferdinando;
- Sand, David J;
- Schulze, Steve;
- Spyromilio, Jason;
- Valenti, Stefano;
- Vinkó, József;
- Wang, Xiaofeng;
- Wheeler, J Craig;
- Yaron, Ofer;
- Zhang, Jujia
The nondetection of companion stars in SN Ia progenitor systems lends support to the notion of double-degenerate systems and explosions triggered by the merging of two white dwarfs. This very asymmetric process should lead to a conspicuous polarimetric signature. By contrast, observations consistently find very low continuum polarization as the signatures from the explosion process largely dominate over the pre-explosion configuration within several days. Critical information about the interaction of the ejecta with a companion and any circumstellar matter is encoded in the early polarization spectra. In this study, we obtain spectropolarimetry of SN 2018gv with the ESO Very Large Telescope at-13.6 days relative to the B-band maximum light, or ∼5 days after the estimated explosion-the earliest spectropolarimetric observations to date of any SN Ia. These early observations still show a low continuum polarization (≲0.2%) and moderate line polarization (0.30% ± 0.04% for the prominent Si ii λ6355 feature and 0.85% ± 0.04% for the high-velocity Ca component). The high degree of spherical symmetry implied by the low-line and continuum polarization at this early epoch is consistent with explosion models of delayed detonations and is inconsistent with the merger-induced explosion scenario. The dense UV and optical photometry and optical spectroscopy within the first ∼100 days after the maximum light indicate that SN 2018gv is a normal SN Ia with similar spectrophotometric behavior to SN 2011fe.